1. Field of the Invention
The present invention relates to methods and apparatus for handling a tubular string. More particularly, the invention relates to a method and apparatus for controlling a string elevator and a spider that handle and support the tubular string.
2. Background of the Related Art
When installing tubular strings, such as casing, the tubular string is alternately supported by a string elevator and spider. The string elevator is controlled by a driller and raised or lowered by a drawworks. The string elevator may include a set of slips that selectively grip or release the tubular string. The spider is controlled by a spider operator and also includes a set of slips that releasably grip or the tubular string, but the spider is not capable of being raised or lowered under the load of supporting the tubular string. Therefore, the stepwise advancement of a tubular string into a borehole requires coordinated use of the string elevator and the spider. Specifically, the spider supports the tubular string while an add-on tubular segment is coupled to the tubular string, and the string elevator supports the tubular string as the drawworks lowers the tubular string further into the borehole. This general process is repeated until the desired length of tubular string has been made up and run into the hole.
However, this process is greatly complicated by the immense load of the tubular string, the extent of damage that can be caused to the rig and the borehole by mishandling the string, and the tremendous investment of equipment and labor involved in operating the rig. The handling of the tubular string must be reliable, efficient, and safe at all times.
One specific challenge in the process of running the tubular string into the borehole is the need to get the internally threaded “box-end” at the proximal end of the tubular string as close as possible to the spider before transferring the load of the tubular string from the elevator to the spider. When the string elevator releases the string and an add-on tubular segment is being coupled, the threaded box should be easily accessible to the crew on the rig floor for making up the threaded connection without the need to scaffold up above the rig floor. Even with a flush mounted spider, the threaded box can be unsuitably high unless the string elevator is controllably lowered to an elevation that invades the operating zone of the spider.
Specifically, the spider typically includes a timing ring that is coupled to the slips within the spider and extends upward as much as two feet above the spider body when the slips are retracted to disengage the tubular string. When the slips of the spider are set to engage and grip the tubular string, however, the timing ring is substantially retracted into or immediately adjacent to the spider body. This “operating zone” defined by the range of vertical movement of the timing ring above the spider presents an opportunity to further descend the string elevator, and thus the threaded box, if the final portion of the descent of the elevator is properly coordinated with the setting of the spider slips. In fact, a driller and spider operator work hard to coordinate control of the elevator and spider so that the elevator can at least partially descend into the operating zone as the spider is actuated.
For example, when the driller lowers a tubular string into the borehole with the string elevator, instead of stopping right above the timing ring on the spider, the driller may continue to lower the elevator. At the same time, the spider operator may actuate the slips on the spider just before the arrival of the descending string elevator to vertically lower the timing ring. The driller may continue the descent of the string elevator such that it follows the timing ring downwardly through at least an upper portion of the operating zone until the spider slips are set on the casing string. As a result, the height of the internally threaded box-end on the proximal end of the tubular string is lowered by one or two feet more than if the string elevator simply stops short of the operating zone of the timing ring. This critical distance makes subsequent operations easier and safer by positioning the connection to be made up near the rig floor.
It is critical during this type of procedure that there be good communication between the driller (controlling the string elevator) and the spider operator. If communication fails, the string elevator may collide into the timing ring on the spider and result in damage to one or both pieces of equipment and lost rig time. Accordingly, the process is subject to operator error.
What is needed is a device and method for enabling the strategic and coordinated handling and positioning of the tubular string using the string elevator and the spider to facilitate the make up of the threaded connection between the proximal end of the tubular string and an add-on tubular segment. What is needed is a device and method for reliably and optimally positioning of the proximal end of the tubular string when the spider slips are set. It is desirable for the device and method automatically setting the spider slips when the string elevator has been lowered to a predetermined position in close proximity to the spider. It is also desirable to have a detector for reliably determining when a descending string elevator is close to contacting the spider.